Optical constants, critical points, free carrier effects, and phonon modes of GaAsN single layers and GaAsN/InAs/GaAs superlattices

Spectroscopic ellipsometry (SE) is employed to study the optical properties of tensile (compressive) strained GaAs_1-yN_y/(InAs)/GaAs [0% ⩽ y ⩽ 3.3%] superlattices and GaAs,sub>1-yN [0% ⩽ y ⩽ 3.7%] single layers for photon energies from 0.75 eV to 4.5 eV and for wavenumbers fiom 100/cm to 600/cm. We provide parametric model functions for the dielectric function spectra of GaAsN in both photon energy ranges. The model functions for photon energies fiom 0.75 eV to 4.5 eV excellently match dielectric function data obtained ftom a numerical wavelength-by-wavelength inversion of the experimental data. Critical-point analysis of the ellipsometric data is performed in the spectral regions of the fundamental band gap and the critical points E_l and E_l+Δ₁. The band-gap energy is red shifted whereas the E_l and E_l+Δ₁ transition energies are blue shifted with increasing y. For y < 1.65% the observed blue shift of the El energy is well explained by the sum of the effects of biaxial (001) strain and alloying. The GaAsN layers show two-mode behaviour in the infrared spectral range (100/cm to 600/cm). We detect the transverse GaAs- and GaN- sublattice modes at wavenumbers of about 267/cm and 470/cm, respectively. The polar strength of the GaN TO mode increases linearly with y, but with different slopes for the GaAsN/GaAs and GaAsN/InAs/GaAs superlattices, respectively. This is due to the different strain states. This effect can be used to monitor strain or nitrogen composition in GaAsN layers. We further detect free carriers in the GaAsN sublayers of the GaAsN/GaAs superlattices. The absence of free carriers in the InAs/GaAsN sublayers goes along with an improved morphology, which is reflected by a decrease of all broadening parameters and by room-temperature photoluminescence emission.